There has been a great deal of interest in developing better and more efficient methods for storing energy for applications such as radio communication, satellites, portable computers, and electric vehicles, to name but a few. Accordingly, there have been recent concerted efforts to develop high energy, cost effective battery cells having improved performance characteristics.
Rechargeable, or secondary cells are more desirable than primary (non-rechargeable) cells since the associated chemical reactions which take place at the positive and negative electrodes of the battery are reversible. Electrodes for secondary cells are capable of being regenerated (i.e., recharged) many times by the application of an electrical charge thereto. Numerous advanced battery systems have been developed for storing electrical charge. Concurrently, much effort has been dedicated to the development of electrical battery chargers adapted to apply charging currents to these various battery systems.
As electrochemical cell chemistry became more complex, design and fabrication of complementary battery chargers did so likewise. As a result, present day, state-of-the-art battery chargers typically have many electronic components, which contribute to potential device failure. Further, increased component count yields increased circuit complexity, further adding to potential failure.
In the past, typical constant current sources for off-line battery charging devices used standard digital/analog amplifiers to select between different constant currents. This approach, while valid, required the use of numerous electronic components, and complex algorithms to select the digital output from a microcontroller, and hardware equivalent circuitry. Further, the prior art approach also imposed a threshold voltage below which operation was impracticable. Accordingly, a wide-band constant current control circuit having a "rail-to-rail" dynamic range was not possible. An example of this type of prior art device is illustrated in FIG. 1.
FIG. 1 illustrates a prior art current control circuit 10, including an amplifier 12, having the output electrically coupled a bipolar transistor 16, via resistor 18. The bipolar transistor 16 is electrically coupled with, and provides signals to an opto-coupler 19. The bipolar transistor 16 may be, for example, an NPN transistor. It is well-known by those of ordinary skill in the art that it is necessary to provide a bias of approximately 0.6 Vdc at the base of a bipolar transistor in order to ensure operation. In designing a constant current source having a dynamic range of, for example, between 0.1 and 2 amps, it becomes apparent that it is difficult to control bias of the circuit below 0.6 Vdc on the transistor. If a ratio of one-to-one between the reference voltage and current exists, it is only possible to control the current between approximately 600 ma and 2 amps. Current control is lost at currents below about 600 ma. Accordingly, it is difficult to control the opto-coupler 19 at currents below about 600 ma. It is possible to change the control ratio in order to address this problem, however, other limitations in the electrical control of the circuit are then introduced. These limitations include non-linearity in current control, and minimal overall current range control.
Accordingly, there exists a need to provide a wide band constant current source for use in a rechargeable battery charging device. Such an improved battery charger should minimize parts count, minimize circuit complexity, and provide for improved dynamic current range.